Screening device



United States Patent lnventor Renzo Sponza Via Querini l7, Mestre, Vcnezia, Italy Appl. No. 721,437

Filed Apr. 15, 1968 Patented Dec. 22, 1970 SCREENING DEVICE 1 Claim, 9 Drawing Figs.

U.S. Cl 210/394, 210/408 int. Cl. B0ld 33/06 Field 01' Search 273;

..... 209/ 21o/41s,40s,393, 394, 216,217,

[56] References Cited UNITED STATES PATENTS 2,857,053 10/1958 Schmiedel 210/394 3,401,801 9/1968 Wedemeyer et a1. 210/394 Primary Examiner-J. L. DeCesare Attorneys-Guido Modiano and Albert .Iosif ABSTRACT: Device for screening pulverulent material with a drumlike screening panel, an impeller member within the space surrounded by said screening panel, the screening panel being supported either rotatably or rigidly; the device comprising further duct means for introducing the material to be screened and circulating air, as well as chambers for collecting the screened and nonscreened material and pneumatic brushes cooperating with the screen.

PATENTEU DEB22 I976 SHEET 1 [IF 4 PATENTEUDEC22|970 SHEET 3 BF 4 INVENTOR ron2omt gh disadvantages.

SCREENING DEVICE I This inventionrelates to a screening device for solid pulverul ent materials,more particularly, a screening device comprising sifting" or screening means having substantially the shape of a surface of revolution or of a drum, I I

it is known that at present the conventional screens employed for screening solid pulvcrulent materials maybe classifiedasfollow's: I I

1. Screens with a flat, slightly inclined, oscillating sifting or screening panel, in which the material to be screened passes over the entire sifting surface as a result of motion I imparted tothe materialby the screen. 2.' Screens with a flat, inclined sifting panel in which the material to be screened movesover the screening surface as a result of vibrations imparted by," for example, electromagnetic devices which cause the screen to vibrate perpendicularly t'oits own axis. Such vibration is intended to facilitate the separation of the Iundcrsi'ze. 3. Cylindrical panel screens which may be vibrated or cooperate with rotating blades-arranged adjacent the 'sifting surface and'having a lowfn'umber of revolutions per unit time. Such blades are arranged to rotate in the same direction as the cylindricalpanel and are designed to centrifuge the solid pulverulent material on said panel and to displace such material in essentially mechanical manner.

Said known screen's, whilst-widely used, are subject to varimonly occurs as a result,'for"examp le,' of electrostatic charges,

that some of the material to be screened accumulates in the sitting mesh, thus decreasing the screening'efficiency.

Saidknowri screens are also generally. subject to the inconvenience that, during screening, an oversize is formed which is greater than the waste proper. u

It is known, that various new and special screening devices have been suggested in order to overcome such disadvantages. These new screening devices havenot, however, found wide application since, while overcoming some oi the disad-. vantages of traditional screening devices, they give rise to new Thus,'for instance, the present applicant suggested a screening device essentially comprising a cylindrical screen rotating about an axial driving shaft, and a fan keyed on said'shaft and arranged internally and coaxially with respect to said cylindri cal screen, said fan being preferably arranged 'to rotate in the opposite direction to said screens Saidfan was intended to project the solid pulverulent material tobe screened against the sifting surface. 3

Whilst a screening device of this latter type satisfactorily overcomes the described disadvantages of the traditional screening devices, it was later discovered that the sifting action was limited to only a portion of the available sifting surface, and the structure of the screening device was relatively complex'and devoid of the elasticity-usually required of such an apparatus. I

The. main object of this invention isthat-of providing a screening device which, apart from being provided with means designed to pneumatically convey the material to be screened in essentially circular manner by means of the fluid-dynamic forces created by an impeller arranged inside a screening panel having the shape of a surface of revolution, further coming panel.

form, granulometry, specific weight", and the like) of the material to be screened not necessitate the turning of the screening panel, the sifting may beieffected solely by means of the passage of air through the sifting panel and the centrifugal force exerted on the material. Such centrifugal force causes the material to-be screened to enter the mesh of said screening I panel, this centrifugal force being set up because, the'material is caused to rotate within the screening panel by the impeller.

Another object of the invention is that of providing a screening device which, where sifting and'constructive conditions allow, may more readily and economically be produced by employing a main impeller of the device at a single or two suctions, since for a sifting effected with full usage of the sifting surface and without useless oversizing, it is sufficient:

1. that the material comes into. contact with a screening mesh adjacent an axial section axiallysituated on the side opposite to that on which the "oversize material is discharged I v 2. that the material is arranged along screening mesh centrally in an'axial direction opposite to that of the axial component of the air sucked at the center of the suction section of the impeller on side of the oversize outlet.

Another object of the invention is that of providing a screening device of the described type in which it is possible to reduce the power and dimensions of the impeller, sincethis is notoriously a special and consequently costly apparatus. This I 'may be achieved by'employing fans for increasing the flow of device in which the material to be screened is inserted, or I downstream of the collection chamber for the screened and I waste material. Alternatively, the de'sired flow of radial air may be obtained by employing equal pressure sources connected to said air inlet, said collection chamber and said sift- A further object of the balance the air pressures at the inlets of the two sections of the impeller, so that, if the air pressure'at one of said inlets should vary, the air pressure at the other inlet will correspondingly be adjusted. One manner of achieving this object (whichdoes not, however, exclude'other per'se known methods) is to arrange said two air inlets of the impeller in the same circuit in :which a single air pressure exists. ltis intended that circuit, as

used herein, may also includethe arnbient external to the screening device, ifv this be at asinglep'ressurey Another method of achieving this object is. to connect said air inlets of the impeller to two sources having a large difference in pressure, so as to be able to balance the fluid dynamic conditions inside the screening device.

A still further object of the inventionis that of providing a screening device whose structure is such as to enable the elimination of dangerous or difficult seals," when such seals are not necessary. According to. another aspect of this object, when seals may absolutely not be eliminated, it is intended to provide special seals which may-readilybe produced, are safe and of inexpensive maintenancef Such special "seals" are pneumatic seals in which the gaseous fluid necessary for the 5 seal is, either derived from a source external to the screening prises means designed to gradually displace the material from the inlet towards the outlet for the nonsifted material. These latter means are intended to enable the material to slide over device or from the relative movement of the parts of the screening device.

I According to'the invention there is; provided a pneumatic screening device which comprises' an-impeller member arranged inside a sifting panel which,- in-determined cases may rotate independently and coaxially about the impeller 'member, said screening device being characterized in that it as great as possible a portion of the sifting surface,.con- I Another object of the invention that of providing a screening device which is of ready and economic construction and which is such that should the characteristics (such as further comprises means arranged to urge, with a fluid-dynamic' and/or. mechanical component, the material which is ar-' ranged inside the sifting panel during the sifting operation.

The invention will more clearly be understood from the folv lowing detailed description of a-preferred embodiment of a screening device for solid pulverulent materials according to invention is that of providing a the invention, illustrated by way of example in the accompartying drawings in which:

FIG. 1 shows a longitudinal elevation section of the screen ing device according to the invention;

FIG. 2 shows a side view of an impeller member employed in the screening device according to the invention;

. FIG. 3-shows a diagrammatic front view of the impeller member of FIG. 2;

FIG. 4 shows a perspective view of a constructive detail of the sifting panel employed in the screening device according to the invention;

FIG. 5 shows a longitudinal elevation section of a modified embodiment of the screening device according to the invention;

'FIG. 6 shows a longitudinal elevation section of another embodiment of the screening device according to the invention;

FIG. 7 shows a partial longitudinal section and elevational view of another embodiment of the screening device according to the invention;

FIG. 8 shows a diagrammatic longitudinal elevation section of another embodiment of the screening device according to the invention;

FIG. 9 shows a plan view along the arrows A of FIG. 8 of the embodiment of FIG. 8.

With reference to FIG. I the screening device according to the invention comprises a sifting panel 1 which is connected to an end of a hollow driving shaft 2 which is supported, with the interposition of roller bearings 3, by the framework 4 of the screening device. 0n the free end of said shaft 2 is keyed a pulley, diagrammatically indicated at 6, which is connected by per se known means to a driving unit (not shown).

Internally and coaxially to said hollow shaft 2 is arranged, with the interposition of roller bearings diagrammatically indicated at 7, a second shaft 8. On one end of said shaft 8 is mounted a pulley 9 which is connected by per se known means to a driving unit (not shown). On said shaft 8 and internally of the space 11 defined by said sifting panel l, is keyed an impeller l2. Said sifting panel 1 is enclosedin a chamber 13 whose lower portion 15 has the'form of a hopper or container. Said hollow shaft 2 is provided, in an intermediate section thereof, with a plurality of through slots 16 which place the interior of said shaft 2 in communication with a pipe 17. Said pipe 17 terminates with an air pipe 18 and a feed pipe 19 for feeding the material to be screened. After passing through the hollow shaft 2 the material to be screened arrives in said space 11. In said spacell, the impeller 12 projects said material, together with air, against the sifting surface of said sifting panel 1. Near the internal wall of said sifting panel 1, said material receives a circular motion as a result of the fluiddyriamic tangential component produced by said impeller 12. The fluid-dynamic radial component produced by the impeller and the centrifugal force due to the motion of the material, tend to cause said material to pass through the mesh of the sifting panel. At the same time, said material is subject to a force having a nonzero component along the axis 3 of the impeller 12. This force gradually causes the material to travel towards the "oversize" outlet and enables the material to pass over the entire sifting mesh before being discharged into the chamber 20 which is designed to receive the oversize" material. Said force is produced due to the fact that the inlet and outlet edges of the blades of said impeller 12 travel along a surface of rotation with different radii with respect to their axes of rotation (which coincide with the axis of rotation of the impeller). After passing through the sifting surface, the screened material falls into the underlying collection hopper or container 15, from whence it is subsequently discharged from the screening device. As said, the oversize material is discharged from the screening device through the pipe 20 which is separated from said hopper 15.

In an alternative embodiment of the screening device according to the invention, the sifting panel does not rotate. An example of such a sifting panel (which might equally well be connected to a vibrator of any type) is shown in FIG. 5, and

may readily be understood from the indications given for FIG. 1. In fact the embodiment of FIG. 5 only substantially differs from that of FIG. 1 in that the sifting panel 1 is not rotatable, whence the main simplifications of the elimination of hollow driving shaft 2 and the seal 29.

In a further embodiment of the screening device according to the invention, the main impeller 12 of the screening device has a single suction stage, as shown in FIG. 6, which may be easily understood from the description of FIG. 1. In fact the only difference between the two FIGS. is that the impeller 12 has a single suction indicated in 60in FIG. 5, whereas the inlets 18 and 19 for the material to be screened and for the air disappear, these inlets being provided in this modification, for instance, by the duct 61 and a suction inlet 62 for the auxiliary air, which is sucked by the impeller itself.

The material, which is sucked by the impeller 60 through the said ducts (which could be for instance, conveyed also by a screw feeder) impinges against the disc 25, which substantially prevents from egressing at the other side of the impeller. The material centrifuged together with the air sucked by the impeller itself, towards the screening panel or cage 1 and then, after having reversed the direction of its axial movement with which it reaches the disc 25, by virtue of the already described means provided for the purpose, it is gradually pushed towards the chamber 20, while rotating circularly at the same time within the interior and in front of the cage 1, thereby being subjected to the screening action.

According to a further modification of the device according to the invention, the main impeller 12 of the screening device has no blades in the sector comprised between the disc 25 and the bottom of the support of the cage 1, Le. the element 26' of FIG. 1'.

Within this sector the bottom 26' and the cage 1 will have preferably a frustoconical shape, which is radiused with the cylindricalportion of the cage 1 and the duct 18 will be connected with the outside pressure source. Since the cage 1 rotates at a speed lower than that of the impeller, the material introduced in the device through the inlet 19 will reach the cage 1 without passing through the impeller 12 since it is entrained only by the air-current coming from the element 19 and by virtue of the light centrifuge effect of the bottom 26'. Consequently the frustoconical or partially frustoconical shape of the bottom member 26' or the radiusing thereof with the cage 1 may be considered as equivalent since both the above mentioned shapes have, in a more or less similar manner, substantially the same effect in reducing the impact component perpendicular to the surface against which the material conveyed by the shaft 2 is urged towards the screening cage 1. I

As a result of repeated and controlled experiments carried out on a prototype of a screening device according to the invention, it has been found to be advantageous, as previously indicated, to provide means to stress, by means of a component parallel to the axis of said impeller 12, the material which, during the sifting operation, is arranged inside the sifting panel. As pointed out, such stressing is intended to enable the exploitation of the entire available sifting surface during the screening. That is to say, the purpose of such stress is to ensure that the material to be screened comes into contact with the sifting surface immediately after being fed into the screening device and thereafter passes over the entire available sifting surface. In order to cause the material to be screened to behave in such a manner, it is necessary to subject it to an axial component at least along a section of the sifting surface. Advantageously this axial component is determined in situ and that is adjacent the sifting panel 1.

When it is desired to employ an embodiment of the screening device according to the invention in which sifting panel is arranged to rotate, the speed of rotation is selected so as to determine a pressure acting from the interior of said sifting panel towards the mesh of the sifting surface. This is designed to facilitate the passage of the material through the sifting surface and limit or cancel out electrostatic forces tending to oppose such passage, and thus increasing the productive efficiency of the screening device. The speed of rotation of the sifting panel should also be'such as to produce a centrifuge action sufficient to cause the displacement of any sifted material adhering to the external wall of said sifting panel,

'As a result of these findings, it was possible to employ means (which will be described hereinafter) producing substantial forces, even opposite forces, whose values were naturally established so that the path of the material would only depend on their resultants and not be disturbed by other extraneous forces. The presence of such extraneous forces may, for instance, be due to charges of conditions, variations in the flow of material, variations in the instantaneous distribution of the material on the sifting panel and the like. In order to satisfy this requirementmamely a better utilization of the sifting surface, various means may be employed. For example, 'a special impeller structure or pneumatic jets or brushes may be arranged inside the screening panel; Alternatively, pneumatic brushes or other means may be arranged outside said screening panel, which will be described hereinafter. .In relation to the impeller, the proposed object is achieved when the angle of the outlet 2 of the blades-of the impeller 12 varies along the entire outlet edge of each blade. Itis in fact known that, by varying the outlet angle of the blade,'the pressure produced by the impeller in that point will be'varied and, consequently, this variation of outlet. angle involves the. presence of values different from the pressure generated by the single constitutive elements into which a blade travelling in direction parallel to nonzero axisof rotation of the impeller 12 may be considered to be divided. The difference in values of said pressure causes the production of forces (fluid-dynamic thrusts) having nonzero componentsparallel to the axis of rotation of the impeller. Such forces are indicated by reference numeral-21 in FIG. I. A similar result may be obtained by varying the inlet angle of the blades of the impeller 12 instead of the outlet angle. Alternatively, the, impeller may be constructed with blades whose outlet edge (FIG. 2) produces, on the surface of rotation created by the same rotation of the impellerabout its own axis, lines which'make an angle y('y 0) with respect to the axial generatrix. Also in this case the forces, with axial determined by the motion of the impeller which is not constant along all the length of the impeller, or a radius r of the surface of rotation determined by the motion which the inlet edge of the blade completes during the movement of the impeller, not constant along all the length of said impeller. In

relation to the achievement of the object in question by employing means arranged externally of the sifting panel 1, the problem is advantageously resolved by using pneumatic brushes (diagrammatically indicated in 22). These pneumatic brushes 22 are designed not-only to unblock or clear the siftof the direction and value of said'axial component is naturally decided in relation to the characteristics of the material to be sifted. Said pneumatic brush 22 comprises, in one embodiment, a tubelike member in which slots 23 are provided. Said slots 23 are inclined with respect to the axis of the sifting panel l, and the thickness of said tubelike member is determined so that said slots 23 have a length'such as to provide the air blades," which pass therethrough, with the desired direction.

The tubelike member, which advantageously has a closed end 22, is supplied with air or other fluid passed through the pipe 22a connected to an'air source (not shown) and is arranged adjacent the sifting panel 1 with said slots 23 directed towards the panel. At least one of said slots 23 is orientated so as to provide said fluid with a nonzero axial component (indicated with reference numeral 24 in. FIG. 1). This type of brush will hereinafter be referred .to as pneumatic brush with orientated slots. According to another embodiment of said pneumatic brushes, there is provided a thin-walled tubelike member provided with a plurality of nonorientated slots directed towards the sifting panel and'having suitable axial lengths.

Using said pneumatic brushes with orientated slits, and by arranging the single slots at different inclinations, the fluiddynamic action of the brush, which is variable from one point to another of the brush, is provided with an axial component which is the resultant of the axial components of the various air blades" passing through said orientated slots. This resultant axial component may readily be adjusted to accord with the characteristics of the materialJThat is to say, said resultant axial component may either act in the direction of movement of the material to be screened, or may act in the opposite direction. It will be evident that, in the latter case, such resultant axial component will be arranged to have an effective value less than the axial component attributed to the movement of the material by the impeller 12, or less than the difference of pressures of the two sections of the impeller. In fact, the value of the resultant axial component of the orien tated brushes should be such that the material to be screened is always urged axially towards the discharge 20 for the oversize. In this manner the amount 'of oversize is reduced, with obvious increase of screening efficiency. The above-described means may either be used singly or in combination (in which case they may have opposite actions to one another).

During the said experiments carried out on a prototype screening device, it was also found to be advantageous, in order to prevent the entrainment of fine material by the oversize material, to provide a transverse sector disc 25 dividing the volume defined by the sifting panel into two contiguous chambers. It has been established that it is not necessary for said sector disc to extend to the outlet edge of the impeller blades, i.e. for the disc 25 to divide said volume into two noncommunicating chambers. Indeed, in certain cases, the fact that said sector disc 25 does not extend to the outlet edge of the blades, means that a more gradual composition of the flows arriving from opposite axial" directions occurs. Moreover, such fact enables a constructive simplification of the impeller blades which may be produced more economically in a single piece and then be welded more readily to said disc 25.

It will be evident that a pneumatic brush with orientated slots of said type may be replaced by one or more nozzles and/or orientated slits," opening externally of the sifting panel and directed towards the sifting'mesh, with an axial nonzero component with respect to the impeller. This is because the effect of such nozzles and/or orientated slits is identical to that of said brushes with orientated slits, the,only difference being in the member which provides the air pressure. Consequently, hereinafter said terms pneumatic brush with orientated slits and one or more nozzles and/or orientated slits will be used interchangeably.

It is also possible to provide .the material to be screened with a movement having a nonzero axialcomponent by providing the sifting panel with a surface of revolution shape which does not have a constant radius alongthe entire axis of said surface of rotation. In this manner,.the material is subject to a cen- Yet another method of producing said axial movement of the material, is to employ the energy of an air pressure source which, through nozzles and/or orientated slits, passes inside the sifting panel or between the impeller and the panel or, al-

I ternatively, in the zone determined by the volumes of rotation defined by the inlet edges of the blades of the two sections of the impeller or adjacent said sections.

The desired axial thrust to be imparted to the material to be screened may also be produced by causing a different pressure I in the chamber 13 for the sifted material. It will be sufficient chamber 13 towards a subsequent container or pipe, in the direction of the wall which separates said chamber 13 from the oversize material chamber, and by suitably narrowing the cross section of the chamber 13 and/or supplying said chamber 13 with air from an external air pressure source. This air is introduced into the chamber 13 axially from the side opposite that in which the outlet for air and oversize is arranged and is consequently not directed towards the sifting surface. In this manner the previously described axial thrust will be obtained.

Further improvements, which tend to render the operation of the screening device according to the invention more simple and guarantee perfect sifting absolutely free from contamination, were shown to be extremely useful during said prototype experiments. In fact since, for some products, it is not necessary for the sifting panel to rotate, it has been found that, in order to eliminate the considerable dangers of difficult seals, it is essential, either for rotating or. nonrotating sifting panel, that the sifting panel, defined as the surface rotation unit covered essentially by the sifting mesh (apart, of course, from any mechanical members necessary to keep the panel together), be connected, in the part thereof which is axially arranged on the side impregnated by the material, to a mechanical member. During the working of the screening device, the sifting panel should not have any uniform sliding motion relative to said mechanical member. The sifting panel may how- ,ever, have other motion relative to said mechanical member,

such as, for example, vibrations. In the latter case the seal question does not represent any problem, even in this point of the device (which is in the path of almost all the material to be screened).

After numerous controlled tests on the screening device according to the invention, it was found to be useful to obtain seal 29, asshown in FIG. 1, in a pneumatic manner, for the same (FIG. 7) being provided between an annular member 32 arranged to rotate about its own axis of rotation and secured to the screening panel and a fixed member connected to, or integral with, the sidewalls outwardly delimiting the respective chambers and 13 for receiving the oversize material and screenedmaterial, respectively.

To this end at least one substantially annular chamber 35 s will be provided between said members, said chamber being the pressure and the delivery of the fluid entering the chamber '35 from an external pressure source (not shown). Such external pressure source is connected to the chamber 35 through at least one canal 50 in manner such that the pressure in said chamber 35 is at least greater than that of the chamber 20 (in order to avoid the flow of oversize material towards the sifted material chamber or at least greater than the pressure in the sifted material towards the oversize material chamber, in the case in which the sifted material is to be better safeguarded from elements of different (smaller) granular form). Altematively the pressure in said chamber 35 should be greater than the pressure of both the chambers 13 and 20,

A particular pneumatic seal which has been found to be very efficient and economic is diagrammatically illustrated in FIGS. 8 and 9.

Such seal is characterized in that it is obtained by securing an additional impeller to the side of the screening panel 1 cpposite the material inlet side. Such impeller is able:

l. to suck (given the position in which it is arranged) clean air which at most contains already screened material (if this is the material of which granular size has to be ensured) or at most containing oversize material (if this is the material of which granularsize has to be ensured) 2. to suck this air at a pressure safely higher than that provided in chambers 13 and 20 by impeller 12 of the screening device to which it is rigidly secured.

3. to supply this air between two members between which the air tightening has to be provided, i.e. between a member arranged to rotate and secured to the screening cage 1 or integral therewith and a fixed member secured to or integral with the sidewalls delimiting the respective chambers for collecting screened material 13 and oversized material 20.

With reference to the specific embodiment shown in FIGS. 8 and 9,-the said impeller is of a particular form in that, in addition to being provided with radial channels 38, which, as already stated, supply air under pressure inside the seal 29, it is further provided with axial channels 37 which permit the oversize material to pass from annular space 36 between impeller 12 and screening cage 1 to collecting chamber 20 for the overme material. In the above mentioned embodiment the said channels are provided in alternating angular sectors of the impeller 33. p

The air sent to the seal 29 by impeller 33 is separated into flows 40, 41 and 42 respectively towards chambers 20, 36 and 13. As the said impeller 33 has a larger diameter than the maximum diameter of impeller 12, the pressure thereby created will be higher than that in the chambers 20, 36 and 13, respectively and the flows 40, 41 and 43, therefore, will occur as indicated in FIG. 8 as soon as the sucking pressures of the two sections of impeller 12 and impeller 33 become substantially equal to one another.

Owing to the substantial amount of air caused to circulate by the impeller 33 and the right pressure value proportional to t the required value thus obtainable, it is possible to make the seal while substantial gaps or spaces exist between the various elements (and this results in a remarkable saving due to smaller degree of precision required during manufacture), bearing in mind that a possible increase in the air flow is then obtainable by a simple proportional increase in the axial dimensions (otherwise reduced) of impeller 33.

A labyrinth such as that defined by elements 43, 44 will increase the already safe seal and completely eliminate the disturbance of secondary centrifugal effects due to the viscosity of the air between the member32 and the wall of the impeller 33 which slides parallel to said member 32 during the relative rotation between the impeller 33 and the sifting panel or cage 1. A similar labyrinth member arranged between the impeller 33 and the fixed part of the screening device, which are in side-by-side relationship to form a canal in which the flow 40 travels, has the same advantage.

It is evident that the efficiency of the labyrinth is considerably increased when the air which passed into the labyrinth from the impeller 33, and which egresses into the annular chamber 36, has the same direction of motion as the axial component which flows from the annular chamber 36' towards the suction of the impeller 33. Such material arrives from the oversize material chamber 20 through the space between the surface of rotation defined (FIG. 8) by the portion of section 45 and the fixed portion 48 of the screening device adjacent said surface of rotation. Such canal system is rovided in conjunction with an air inlet 51 of the impeller 33 situated on a diameter sufficiently less than that of the inlet edges of the section of the impeller 12 adjacent the impeller 33, and/or axially shifted in the direction of the path of the air sucked by said section adjacent the main impeller 12. Such arrangement fully guarantees that the air sucked by the impeller 33 of the seal 29 is free from any material to be sifted.

For reasons of ease of maintenance it has been found to be advantageous to produce the sifting panel or unit in the manner shown in H0. 4. Such sifting unit is formed of a frame which comprises a disc base la, connected through arms lb, to an annular element 1d. On said frame are externally and releasably mounted the sifting panels 1 proper. On said panel 1 is fixed a net 1f having mesh of preestablished size. The mesh of said net If is determined in relation to the characteristics of the screening to be carried out. This arrangement is advantageous, taking into account the fact that the sifting unit is designed for use in the screening of materials which are difficult to sift, when considerable, and also opposite, air currents and centrifugal forces are employed. It will often be necessary, consequently, to replace the nets, especially due to I wear and the high potentiality desired.

For particular types of material, and in determined occasions, it has been found to be desirable for the shifting unit to also have a grinding action on the material to be screened. In such case, one of the parts of the sifting panel normally occupied by a net, will instead be provided with surfaces of per se known type (since centrifuge grinding systems are well known). Such grinding surfaces have projections or simply roughnesses, or a coefficient of friction between material and surface similar to the intrinsic coefficient of the said material in working conditions. Said grinding surfaces determine a grinding action on the material which is pushed against said surface by centrifugal force, and caused to slide along the latter. In this way a grinder and a sifter are provided together in a single screening device, both working due to fluid-dynamic action and centrifugal force.

During tests carried out, in order to reduce the power and dimensions of the impeller which, as previously pointed out, is a special and consequently costly apparatus, it was found useful to favour the flow of radial air by means of the use of nor mal fans arranged externally of the screening device and in series with any one or more of the air inlets and/or outlets of the device. It was also found to be important to balance the pressures of the two impeller air inlets. It was further found to be extremely important for the air supplies to the two sections of the impeller to be connected either to a single source (which may be the ambient atmosphere), or to two members having a large relative difference of pressure, in order to be able to balance the fluid-dynamic conditions inside the device according to desire.

It is clear that, independently of the choice of these specific auxiliary means, it is possible to cause an axial component directed from the sections of the impeller, on the material inlet side, towards the oversize outlet side, as soor i a the z average static pressure energy P or kinetic energy 5%.

panel 1, is less than that average P or present in the volume thus defined:

establish the desired balance of ressure.

It will be understood that, i screening conditions require this, the device according to the invention may be adapted to allow the material to be screened to reach the screening place without being subjected to the centrifugation effect of the impeller through which it passes into the cage 1. For the purpose, it is sufiicient to construct without blade the sector of the impeller situated at the material inlet side, with respect to the disc 25, and the bottom 26 of the cage with a radiused form even of frustoconical shape, joining the cage 1 and to connect this sector with outside sources of pressure fluid for the pneumatic conveyance of material along the considered path.

It will also be appreciated that the oriented slots" such as the slots 23 are equivalent to nozzles" directing the air current egressing therefrom in a desired direction and in the preceding description the mentioned terms are used indifferently.

It will be further appreciated that instead of screening nets or meshes, solid metallic surfaces may be used advantageously in the zone in which the material to be screened impinges the cage 1 after having passed the impeller 12, when such material has a remarkable centrifugal action which could otherwise easily break the screening net.

I claim:

1. A device for screening pulverulent material including a housing, a screening panel having a round shape and defining a first chamber surrounded by said screening panel, and having a first and a second axial end thereof, said screening panel having a supporting frame thereof and being arranged within said housing at a distance from the inner walls thereof to define a second chamber therewith, an impeller member having an axis of rotation within said first chamber, means defining inlets in said housing and communicating with said first chamber for introducing air and the material to be screened therein, means defining an outlet to communicating with said second chamber for discharging the screening therefrom and means defining a discharge communicating with said first chamber for discharging the oversize, a plurality of air jet nozzle means located outside said screening panel and directing jets of air against said screening panel, movably cooperating sealing means on said supporting frame of the screening panel and on said housing for preventing said screenings to mix with said oversize, wherein, according to the improvement, said impeller member comprises a blade supporting disc member at said second axial end of said first chamber, a shaft coaxial with said axis of rotation for supporting said disc and extending therefrom outside said first chamber in a direction opposite to said first axial end thereof, a plurality of circumferentially spaced apart blades fixed on said disc and extending through said first chamber towards said first axial end thereof, said blades having peripheral edges defining a circular path during rotation of the impeller, said means defining inlets comprising duct means arranged near said first axial end and opening into said first chamber near said axis of rotation, said means defining a discharge comprising further duct means arranged near said first axial end and opening into said first chamber near the periphery thereof, and wherein said movably cooperating sealing means comprise pneumatic sealing means at least near said first axial end and including at least one flange shaped ring member extending over at least one peripheral edge of said screening panel supporting frame and at least one channel shaped ring member surrounding said flange shaped ring member and fixed on said housing, and duct means connected to said channel shaped ring member and communicating with a source of pressurized air. 

